<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>点击获取用户数据</button>
</body>

</html>
<script>
    const btn = document.querySelector('button');
    btn.onclick = function() {
        const x = new XMLHttpRequest();
        //因为是同源策略，所有url可以简写
        x.open('GET', '/data');
        x.send();
        x.onreadystatechange = function() {
            if (x.readyState === 4) {
                if (x.status >= 200 && x.status < 300) {
                    console.log(x.response);
                }
            }
        }
    }
</script>